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Astronomers are forecasting a brief shower of Ursid meteors on Thursday night, when the Earth will hit a dense trail of dust created in 1405, before the birth of Columbus. Excellent viewing is predicted over both the United States and Canada.

Dr. Peter Jenniskens of the SETI Institute, who is based at the NASA Ames Research Center in the heart of Californias Silicon Valley, and researcher Esko Lyytinen, from Helsinki, Finland, have used research and observation skills honed during the 1999 Leonid meteor storm to make the prediction. The shower is expected to hit the Earth at 2:29 a.m. EST on Dec. 22 (or 11:29 p.m. PST Dec. 21).

"The normally ordinary-looking Ursids have long puzzled researchers because of thetwo intense showers seen in 1945 and 1986," said Jenniskens. "Both of these very spectacular showers lagged the passage of the comet by as much as 6 years. By the time these meteors hit the Earth, the comet was on its way back to the outer reaches of the solar system, almost as far from Earth as it ever gets."

The Ursid meteors are caused by dust particles ejected from comet 8P/Tuttle that plunge into Earth's atmosphere. The meteors appear to come from the constellation of Ursa Minor (the Little Bear) close by the pole star. In its 13.6-year orbit around the sun, comet 8P/Tuttle never ventures inside Earths orbit. As a result, its meteor shower usually is unspectacular. It has been 6 years since the last return of the comet.

In a paper submitted to WGN, the journal of the International Meteor Organisation, Jenniskens and Lyytinen explain the 6-year lag of the meteor shower, and forecast this years rich display. Once the meteoroids are ejected into space, they say, it takes as much as six centuries before their orbits are sufficiently changed by the planet Jupiter so they can hit the Earth. During that time, the particles slowly fall behind the orbiting comet that produces them. After six centuries, that lag amounts to just about 6 years.

Jenniskens and Lyytinen are the first researchers to link the cycles of intense Ursid showers with a particular passage of comet 8P/Tuttle. The 1945 outburst was caused by dust shed in 1392, while the 1986 shower was dust from 1378, six centuries ago. The researchers calculate that this year, the Earth will pass the center of yet another trail approximately xxxx miles (at a distance of only about half the distance from the Earthhalfway to the moon) from the center of the meteor trail.

On Thursday, the Earth will find in its path the trail of dust ejected in 1405, they say. The shower is expected to last 2 to 3 hours, and possibly reach rates of one meteor per minute. Many of these will be faint meteors, so observers are encouraged to go to a dark location away from city lights for best viewing.

This years Ursids will provide an unexpected bonanza for astrobiologists, the scientists who study the origin, evolution, distribution and future of life in the universe. The shower will enable researchers to probe the composition and morphology of grains from a comet not previously sampled. Like the Leonid meteors, the Ursid meteoroids can be precisely dated, but they also offer rich additional research possibilities are different in important ways because they have spent six times longer in the solar system environment and plow into Earths atmosphere with just half the Leonids' speed.

NASA Ames Research Center is NASAs lead center for astrobiology. The central administrative office of the NASA Astrobiology Institute (NAI), a research consortium involving academic, non-profit and NASA field centers, also is located at Ames. The NAI has international affiliate and associate members.

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